1. Field of the Invention
The present invention relates to a lens apparatus having an automatic focus adjusting function, which is to be used for optical apparatuses such as a television lens and a video lens. The present invention also relates to an image pickup system including the lens apparatus.
2. Description of the Related Art
Conventionally, various proposals have been made as autofocus (AF) technologies to be adopted into image taking apparatuses such as a camera and a video camera. For example, the following automatic focus adjusting method is widely known. That is, beams coming from a subject through different exit pupil regions of an image taking lens are imaged onto a pair of line sensors, and a pair of image signals are obtained through photoelectric conversion performed on subject images, to thereby determine a relative position displacement amount of the pair of image signals thus obtained. Based on the displacement amount, a defocus amount of the subject is calculated, and the image taking lens is thus driven.
An in-focus position of a focus lens based on a subject distance can be determined in the AF system employing phase difference detection, and hence this AF system has such a feature that a focus state may be obtained more quickly than a contrast AF system.
Japanese Patent Application Laid-Open No. 2005-292779 discloses an autofocus system in which a focus lens is moved based on a contrast of a subject image. The autofocus system includes a focus demand that enables manual setting of a driving speed of the focus lens and a cut-off frequency of a filter for detecting the contrast. At a speed designated with the focus demand, the focus lens is driven to an in-focus position.
Japanese Patent Application Laid-Open No. 2009-145645 discloses that a focus lens is driven at different driving speeds in two regions within a movable range of the focus lens, wherein the two regions are different from each other in moving amount of the focus lens relative to a change amount of a focus state. A depth of field (in particular, near-side depth of field) is shallower (smaller) on a close side than on an infinity side in terms of a subject distance, and hence when the focus lens is driven at the same speed, an image blur amount changes slowly on the infinity side while changing quickly on the close side. Thus, there is a difference in manner of change in image blur amount. In order to prevent the above-mentioned phenomenon that the manner of change in image blur amount differs depending on the subject distance, the driving speed of the focus lens is changed in two stages depending on a focus range.
In general, when a depth of focus is defined as a degree of a defocus amount D within which it may be determined that an in-focus state is obtained, the depth of focus may be expressed as 2×Fno×δ, where Fno represents an f-number and δ represents a permissible circle of confusion. Hence, the depth of focus is constant even at any focal length and subject distance. However, the depth of field, which indicates a range within which the subject may be determined to be actually in-focus, is expressed as the following expressions (1) and (2), where d1 and d2 represent a near-side depth of field and a far-side depth of field, respectively, f represents a focal length of the lens and L represents a subject distance.d1=δ×Fno×L2/(f2−δ×Fno×L)  (1)d2=δ×Fno×L2/(f2+δ×Fno×L)  (2)
From the above-mentioned expressions, the depths of field d1 and d2 are smaller as the subject distance L is smaller, and the depths of field are also smaller as the focal length f of the lens is larger.
FIG. 6 shows a relationship between the defocus amount indicating an in-focus degree and the subject distance, and between the defocus amount and the focal length of the lens. FIG. 6 shows a change in defocus amount at the same driving amount of the focus lens. On a wide-angle side on which the focal length of the lens is small, the change in defocus amount relative to the change in driving amount of the focus lens is extremely small. In other words, in the same defocus amount, the driving amount of the focus lens necessary to obtain the in-focus state is larger as the focal length of the lens is smaller.
FIG. 7 shows an example of a focus lens driving locus in a case where a zoom position is in a telephoto side. The axis of abscissa represents time and the axis of ordinate represents a position of the focus lens. In FIG. 7, a depth of focus D and a threshold value 2D of the defocus amount respectively indicate the positions of the focus lens corresponding thereto. The same applies to the description that is given later referring to FIGS. 3, 4, and 8.
During a period from a time t0 to a time t1, the focus lens is driven at high speed in a direction in which the defocus amount is zero. The focus state is monitored as necessary even during the driving of the focus lens, and when the defocus amount reaches to a predetermined value (threshold value corresponding to positive/negative focal depths), the driving speed of the focus lens is shifted to a low speed. During a period from the time t1 to a time t2, in which the defocus amount falls within the range of the threshold value, the focus lens is driven at low speed, and is stopped at an in-focus point, at which the defocus amount is zero. By driving the focus lens as described above, that is, quickly driving the focus lens at high speed and driving the focus lens at low speed in the vicinity of the in-focus point, the in-focus state is obtained with high accuracy without passing through the in-focus point.
Hereinafter, FIG. 8 shows an example of a focus lens driving locus in a case where the driving method for the focus lens on the telephoto side shown in FIG. 7 is applied to the wide-angle side. Similarly to the case of FIG. 7 where the zoom position is on the telephoto side, during the period from the time t0 to the time t1, the focus lens is driven at high speed in the direction in which the defocus amount is zero. The focus state is monitored as necessary even during the driving of the focus lens, and when the defocus amount reaches to the predetermined value (threshold value corresponding to the positive/negative focal depths), the driving speed of the focus lens is shifted to low speed. During the period from the time t1 to the time t2, in which the defocus amount falls within the range of the threshold value, the focus lens is driven at low speed, and is stopped at the in-focus point, at which the defocus amount is zero. However, on the wide-angle side, the depth of focus is large and hence the position of the focus lens within the threshold value of the low speed driving is very distant from the in-focus point. As a result, it takes an extremely long period of time for the low speed driving (from the time t1 to the time t2).
In a case where the threshold value for determining the shift from the high speed driving to the low speed driving is set based on the defocus amount (depth of focus), the depth of field corresponding to the depth of focus is large on the wide-angle side, and hence on the wide-angle side, the focus lens is shifted to the low speed driving at a position that is more distant from the in-focus point than on the telephoto side. Therefore, when the position of the shift to the low speed driving is determined based on the depth of focus, as shown in FIG. 8, it takes an extremely long period of time for the low speed driving (from the time t1 to the time t2) on the wide-angle side, with the result that the focus lens cannot be moved quickly to the in-focus point.